1. Field of the Invention
The present invention relates to the injection of a gas into a liquid, and in particular a liquid metal. More particularly, it relates to the field of metallurgical reactors, such as melting furnaces, converters of pig iron, of alloyed or non-alloyed steels or of non-ferrous materials, as well as the electric arc furnaces used in particular for the production of steel from scrap iron or scrap iron substitutes, and generally to any injection of a gas into a liquid.
2. Description of the Related Art
In the technology of production of steel in an electric arc furnace, scrap iron or its substitutes are melted by establishing an electric arc between the electrodes of the furnace and the metal such as to supply the energy to melt the metal during the melting phase and to keep it in the molten state during the phase of refining the said metal.
During this refining phase, oxygen is used to decarburize the metal and to form a so-called foaming slag by reaction of the oxygen with the carbon produced from the metal or injected at the surface of the metal bath specifically for this purpose.
This injection of oxygen can be carried out with the aid of door lances which are expendable or cooled with water. In this case, the lance is mounted on a mobile component, which involves an employee assigned to its use, and high maintenance. Furthermore, the oxygen is not injected uniformly into the bath, which adversely affects the high performance of the furnace, the metal bath being homogeneous neither in temperature nor in chemical composition.
This injection can also be carried out with the aid of injectors arranged in the wall of the furnace. This arrangement allows the oxygen to be distributed more uniformly in the metal bath and the slag and the thermal yield of the furnace to be increased, which enables the steel production time to be reduced and the air intakes to be reduced due to a better tightness of the furnace (the door of the furnace can be closed due to the omission of the door lance). However, such an injector must be capable of withstanding the high thermal loads without being destroyed prematurely, and capable of injecting the oxygen under conditions such as may be reached in the bath of liquid metal and of penetrating into this. The obvious solution to the person skilled in the art comprises placing the injector of oxygen close to the metal bath to ensure that the oxygen reaches the said bath. However, the closeness of the metal bath causes premature wear of the injector.
To reduce the wear, the person skilled in the art tends to move the end of the lance away from the surface, to the detriment of the penetration of the jet into the liquid metal.
It is known from GB-A 623 881 to use a supersonic injection lance to inject the oxygen at supersonic velocity for the decarburization of steel in a liquid bath. The problem which results from this type of installation is that the jet of oxygen opens out at the exit of the injector nozzle, which reduces the force of penetration of the jet into the bath and increases the risks of splashes.
In order to improve the force of penetration of the jet of oxygen into the metal bath, it is known from Re 33 464, in particular col. 7, lines 32 to 41, to use a supersonic jet of oxygen and to surround this jet with a flame, the envelope of this flame extending substantially up to the surface of the molten metal. Due to the flame surrounding this jet (and its high temperature), such systems are said to be capable of preserving all the coherence of this jet, avoiding disintegration thereof. Such systems are thus said to improve the penetration of the oxygen into the bath.
While in theory it appears obvious to the person skilled in the art that the increase in the temperature around the jet will cause a reduction in the density of the ambient medium and therefore cause an effect of lower resistance of this medium with respect to a cooler medium, which in theory enables the supersonic jet to remain more xe2x80x9ccoherentxe2x80x9d, it has been found, however, that in practice the interactions between the flame and the jet, such as, for example, buoyancy effects, in fact have adverse effects on the jet and reduce the force of penetration. These buoyancy effects are caused by the hot current of a flame in a medium which is colder than this flame. The flame which surrounds the supersonic jet and which passes through a medium at a temperature of about 1500xc2x0 C., while the temperature of the flame is close to 2300xc2x0 C. or higher, thus bends upwards, and interacts with the jet during this ascent, in particular in the vicinity of the bath, precisely where it could have been hoped to preserve the xe2x80x9ccoherencexe2x80x9d of the supersonic jet. This coherence is now in fact destroyed.
It is furthermore known that the use of a flame created by a burner in an electrical metallurgy furnace is an effective complement for supplying energy to the charge, and thus increases the rate of melting. The exchange of energy between the flame and the charge is effective as long as the exchange surface is significant, that is to say as long as the scrap iron has not melted, and the temperature difference between the flame and the charge is high.
The process according to the invention enables these disadvantages to be avoided.
According to the invention, a process is provided for injection of a gas, such as oxygen, with the aid of a nozzle into a liquid metal bath contained in a metallurgical vessel, the said nozzle being installed in the side wall of the said vessel above the metal bath and orientated at an angle xcex1 with respect to the perpendicular, the downstream end of the nozzle through which the gas escapes in the direction of the liquid metal bath being situated at a distance L from the surface of the liquid metal, the said nozzle being fed by the gas which penetrates into the nozzle through its upstream end at a pressure P1, and being ejected from the nozzle through its downstream end at a pressure P2, which process is characterized in that the downstream pressure P2 of ejection of the gas into the metallurgical vessel and the pressure P3 in the metallurgical vessel are connected to one another by the relationship:
0.9P3xe2x89xa6P2xe2x89xa61.1P3 
in that the distance L between the downstream end of the nozzle and the surface of the liquid metal is equal to:
L(meters)=C*{square root over (qe)}xc2x10.15m 
Where   C  =                                          4                          π              *                              P                2                            *              M                                      ⁡                  [                                    γ              RTo                        ⁢                          (                              1                +                                                                            γ                      -                      1                                        2                                    ⁢                                      M                    2                                                              )                                ]                                      -          1                /        4              *          xe2x80x83        ⁢          [              4.2        +                  1.1          *                      (                                          M                2                            +              1              -                                                T                  j                                                  T                  a                                                      ]                    ⁢                                                    ρ                j                                            ρ                a                                                        
Notation:
P2: pressure of the jet at the exit (Pa), which must be equal to the pressure in the metallurgical vessel. In the case of an arc furnace, P2=105 Pa.
M: Mach number calculated according to the following formula:       M    2    =            2              (                  γ          -          1                )              *          [                                    (                                          P                1                                            P                2                                      )                                (                                          γ                -                1                            γ                        )                          -        1            ]      
(this can be taken between 1.5, corresponding to an upstream pressure P1 of 3.7xc3x97105 Pa, and 3.15 for a pressure P1 of 45xc3x97105 Pa).
To: temperature of the gas (K) (in general 294K)
R: ideal gas constant (8.314/molar mass of the gas),
qe: mass flow rate (kg/s)=volume flow rate (Sm3/h)*molar mass of the gas/3600/0.0224 (l/mol)
Tj: temperature at the exit of the nozzle (K)
xcfx81j: density of the jet at the exit of the nozzle (kg/m3), calculated from:       P    2        R    *          T      j      
Ta: temperature of the ambient medium (K)
xcfx81a: density of the ambient medium (kg/m3), calculated from:       P    3        R    *          T      a      
and in that the injection of gas is carried out when the temperature of the gases in the metallurgical vessel is greater than 800xc2x0 C., preferably 1000xc2x0 C.
The gas injected will preferably be chosen from oxygen, nitrogen, argon, hydrogen, carbon monoxide, carbon dioxide, hydrocarbons and, in particular, alkanes, alkenes and alkynes, natural gas and sulphur hexafluoride, these gases being injected by themselves or as mixtures.
According to a preferred embodiment, the velocity of the gas during its ejection by the nozzle will be supersonic. Preferably also, the nozzle for injection of the gas is a nozzle comprising, from upstream to downstream, according to the direction of entrainment of the gas, a convergent truncated upstream part, followed by a cylindrical central part, followed by a divergent truncated downstream part, followed by a cylindrical part which emerges over the atmosphere of the metallurgical vessel, and the angle xcex1 will preferably be between 30xc2x0xe2x89xa6xcex1xe2x89xa660xc2x0, and more preferably xcex1=45xc2x0xc2x15xc2x0.
According to one embodiment of the invention, the process is regarded in that the nozzle for injection of the gas is a nozzle comprising, from upstream to downstream, according to the direction of entrainment of the gas, a convergent truncated upstream part, followed by a cylindrical central part, followed by a divergent truncated downstream part which emerges over the atmosphere of the metallurgical vessel, the ratio of the vertex angles of the convergent and divergent cones respectively being between about 1.5 and 2.5.
According to another embodiment, the convergent cone (21) half-angle is between 2 and 12xc2x0 and the divergent cone (23) half-angle is between 15xc2x0 and 35xc2x0.
According to another embodiment, the process according to the invention is characterized in that the flow rate of gas is between 50 and 5000 Sm3/h, and preferably between 1000 and 3500 Sm3/h.
According to another variant, the process according to the invention is characterized in that the velocity of the flame is between 150 m/s and 300 m/s.
According to one embodiment of the invention, the gas injected into the metal will be a hydrocarbon or a mixture of gaseous hydrocarbons, preferably natural gas.
The injection of gas, preferably supersonic, will be carried out, for example, in alternation with the injection of a flame into the metallurgical vessel from a wall of the vessel in the direction of the solid and/or liquid metal present in the vessel.
According to another variant, the injection of gas will be carried out simultaneously with that of a flame into the metallurgical vessel from a wall of the vessel in the direction of the solid and/or liquid metal present in the vessel, where it will be desirable to obtain simultaneously energy for heating the metal and to carry out an injection of gas into the metal (such as, for example, oxygen for decarburization)
The jet of gas at the exit of the nozzle will be an adjusted jet, that is to say a jet of which the static pressure at the exit of the injector is equal to the pressure prevailing in the atmosphere passed through by this jet, to about + or xe2x88x9210% of this pressure. It has been found that an adjusted jet allowed, for example, a supersonic velocity of the gas to be maintained over the longest distance possible for a given available gas pressure, since the creation of shock waves, which would reduce the kinetic energy of the jet, was avoided in this way. Such an adapted jet can be delivered by an injector comprising a so-called Laval nozzle which has, from upstream to downstream, a converging part, a throat where the velocity of the oxygen becomes sonic, a diverging part where the velocity of the oxygen becomes supersonic and a straight (cylindrical) part for stabilizing the jet of oxygen, that is to say channelling the jet and reducing any turbulence. The said nozzle has dimensions which are a function of the desired flow rate and exit velocity of the jet of oxygen, and the pressure of the oxygen at the intake of the nozzle is regulated such that the exit pressure of the jet is substantially equal to the pressure of the surrounding atmosphere (in the metallurgical vessel, for example). The substantial equality (preferably a difference of less than about 10%) between these two pressures allows the compression-expansion effects, which result in the supersonic jet with all the more intensity the greater the pressure difference, to be limited. These compression-expansion effects here considerably reduce the kinetic energy of the jet, and therefore the isovelocity length (length over which the jet maintains its initial velocity). Thus, for example, if the nominal intake (upstream) pressure of the gas in the nozzle is reduced by 3xc3x97105 P, this results in a reduction in the isovelocity length of about 35%.
The use of the injector in supersonic mode is necessary during the periods of the refining of the charge of molten metal, in particular in the electric arc furnace. However, it can start during the period of melting of the charge. On furnaces using charges based on AIS (xe2x80x9cAlternative Iron Sourcesxe2x80x9d), the supersonic mode can be used during the entire melting and refining.
The ratio of the flow rates of oxygen delivered by the injector in supersonic mode and in burner mode is advantageously substantially equal to 5.
Furthermore, according to the prior art as described in GB-A-623 881, the arrangement of such a nozzle, in particular a supersonic nozzle, on the walls of the furnace was based on empirical rules leading to a very uncertain effectiveness of this device.
An other object of the present invention is to be able to determine, as a function of specific parameters of the furnace, and in particular the flow rate of the gases, the operating conditions of the furnace (temperature and pressure), the available pressure of the gas injected into the nozzle, the maximum isovelocity length of the jet (that is to say the maximum length where the velocity of the gas injected, such as oxygen, remains substantially constant to xc2x110%), and thus to be able to determine the best arrangement of the injector on the walls of the furnace.
According to the invention, this distance L between the nozzle of the injector and the surface of the metal bath would be equal to:
Lm=C*{square root over (qe)}xc2x10.15m 
where   C  =                                          4                          π              *                              P                2                            *              M                                      ⁡                  [                                    γ              RTo                        ⁢                          (                              1                +                                                                            γ                      -                      1                                        2                                    ⁢                                      M                    2                                                              )                                ]                                      -          1                /        4              *          xe2x80x83        ⁢          [              4.2        +                  1.1          *                      (                                          M                2                            +              1              -                                                T                  j                                                  T                  a                                                      ]                    ⁢                                                    ρ                j                                            ρ                a                                                        
P2: pressure of the jet at the exit (Pa), which must be equal to the pressure in the metallurgical vessel. In the case of an arc furnace, P2=105 Pa.
M: Mach number calculated according to the following formula:       M    2    =            2              (                  γ          -          1                )              *          [                                    (                                          P                1                                            P                2                                      )                                (                                          γ                -                1                            γ                        )                          -        1            ]      
(this can be taken between 1.5, corresponding to an upstream pressure P1 of 3.7xc3x97105 Pa, and 3.15 for a pressure P1 of 45xc3x97105 Pa).
To: temperature of the oxygen (K) (in general 294 K)
xcex3: ratio of Cp/Cv, Cp and Cv being, respectively, the molar thermal capacities at constant pressure or volume. For oxygen at To, of the order of 1.4.
R: ideal gas constant (8.314/molar mass of O2)
qe: mass flow rate (kg/s)=volume flow rate (Sm3/h)*molar mass of oxygen/3600/0.0224 (l/mol)
Tj: temperature at the exit of the nozzle (K)
xcfx81j: density of the jet at the exit of the nozzle (kg/m3), calculated from:       P    2        R    *          T      j      
Ta: temperature of the ambient medium (K)
xcfx81a: density of the ambient medium (kg/m3), calculated from:       P    3        R    *          T      a      
In the case of electric arc furnaces:
1.1 less than M less than 3.5 
1000 Sm3/h less than volume flow rate less than 20 000 Sm3/h 
1400xc2x0 C.xe2x89xa6ambient T (Txc2x0 inside the electric furnace during refining) less than 2500xc2x0 C. 
According to another variant of the invention, an injector comprises a nozzle which has, from upstream to downstream, a converging part, a throat where the velocity of the oxygen becomes sonic, a diverging part where the velocity of the oxygen becomes supersonic and a straight (cylindrical) part for stabilizing the jet of oxygen. The said nozzle has dimensions which are a function of the desired flow rate and exit velocity of the jet of gas, the pressure of the gas at the intake of the nozzle being such that the exit pressure of the jet is substantially equal to the pressure of the surrounding atmosphere. The substantial equalness (difference of less than 105 pascals) between these two pressures enables the compression-expansion effects, which result in the generally supersonic jet delivered by this nozzle, to be limited. These compression-expansion effects here reduce the kinetic energy of the jet. It is therefore necessary to limit these effects, resulting in the need for an adjusted jet (that is to say of which the static exit pressure is substantially equal to the pressure of the surrounding atmosphere to plus or minus 105 Pascal).
In order to avoid the creation of shock waves inside the nozzle, the latter preferably has rounded joining surfaces, according to well-defined specifications described below, between, respectively, the converging part and the throat, the throat and the diverging part, and the diverging part and the straight part.
According to another characteristic of the invention, the start of the melting of the metal is accelerated by burning a fuel, such as, for example, a natural gas, during the period of melting of the scrap iron, this natural gas being introduced, for example, through an annular channel surrounding the nozzle, the oxygen necessary for the combustion of the natural gas being injected through the nozzle. During this phase, the oxygen is injected at a much lower pressure at the intake thereof, such that it is ejected by the nozzle at a subsonic velocity. In order to create a stable flame between the oxygen, injected at a velocity of between 100 m/s and about 300 m/s, on the one hand and the fuel, such as the natural gas, injected at a generally much lower velocity, for example between about 30 m/s and 150 m/s, the fuel and the carburizing agent are mixed in a combustion chamber.
This arrangement has, in particular, the advantage over devices and processes in which a flame surrounds the supersonic jet of causing the injector to function in a lance mode (a single fluid injected) or burner mode (two fluids injected) with solely two gas feeds (one for the oxygen, the other for the natural gas) instead of three gas feeds necessary in the known devices and processes.
The processes and apparatuses according to the invention of course have, in addition to the advantages mentioned above, that is to say a supersonic injection, when this is necessary, without a shock wave, thus having a better penetration into the metal jet, the following advantages: for an even penetration into the molten metal, it is possible to locate the end of the lance much further from the surface of the metal bath than a normal lance (without a flame). For an even penetration into the molten metal, it is possible to locate this lance according to the invention at a distance similar to the distance at which a lance surrounded by a flame (such as described in Re 33 464) is placed, without having the need for this flame in order to achieve it, which allows actual savings in gas consumption for the client, for the same outputs.